1. Field of the Invention
The present invention relates to a fuel injection control system for controlling quantity and timing of fuel injection into cylinders of an internal combustion engine such as a diesel engine. Specifically, the present invention relates to a fuel injection control system capable of performing energy changing control for changing a charging amount to a piezo element of a piezo injector.
2. Description of Related Art
A common rail type fuel injection system is used in a diesel engine. In the common rail type fuel injection system, a high-pressure supply pump pressure-feeds high-pressure fuel to a common rail, which is common to respective cylinders. Thus, the common rail accumulates the high-pressure fuel. The high-pressure fuel is supplied to injectors of the respective cylinders from the common rail. The injectors perform fuel injection under control of an engine control unit (ECU). Each injector has a nozzle portion for injecting the supplied high-pressure fuel through its injection hole. An opening degree of the injection hole is changed by a nozzle needle inserted inside the nozzle portion.
A back pressure chamber is formed so that the back pressure chamber faces a back end surface of the nozzle needle, for instance. The high-pressure fuel supplied to the injector is introduced into the back pressure chamber through a restriction and generates back pressure of the nozzle needle. The nozzle needle is opened or closed by changing the back pressure. The back pressure is changed by back pressure changing means. The back pressure changing means has a valve chamber between the back pressure chamber and a low-pressure passage. The back pressure changing means releases the pressure in the back pressure chamber to the low-pressure passage by moving a valve member accommodated in the valve chamber. Lately, a piezo actuator utilizing piezoelectric effect of piezoelectric ceramics and the like is used to drive the valve member.
Conventionally, in the common rail type fuel injection system having a piezo injector utilizing a piezo actuator and the like, an upper limit value of charging voltage applied to a piezo stack is changed in accordance with common rail pressure in order to control the charging amount to the piezo stack at a required minimum value. A method for changing the charging amount to the piezo stack between two levels of a large charging amount and a small charging amount is disclosed in Japanese Patent Unexamined Publication No. 2001-241350, for instance.
However, in the conventional common rail type fuel injection system, which aims to reduce heat generation by controlling the charging energy to the piezo stack at a minimum value, the charging amount to the piezo stack, or the upper limit value of the charging voltage applied to the piezo stack, changes because the charging energy to the piezo stack is changed as shown by a broken line in a part (b) of FIG. 8. A solid line in a part (a) of FIG. 8 shows a waveform of an injection command pulse “PULSE”. A solid line in the part (b) of FIG. 8 shows a waveform of charging energy “Ec”, or the charging voltage, to the piezo stack when the charging energy is small. The broken line in the part (b) of FIG. 8 shows the charging energy “Ec” to the piezo stack when the charging energy is large. A solid line in a part (c) of FIG. 8 shows a waveform of charging current “Cc” applied to the piezo stack when the charging energy is small. A broken line in the part (c) of FIG. 8 shows a waveform of the charging current applied to the piezo stack when the charging energy is large. A solid line in a part (d) of FIG. 8 shows an injection ratio “R” when the charging energy is small. A broken line in the part (d) of FIG. 8 shows the injection ratio “R” when the charging energy is large. A time point ts in FIG. 8 shows start timing of the injection. A time point te in FIG. 8 shows end timing of the injection. Accordingly, a discharging waveform changes with respect to time if the charging amount is changed. The end timing te of the injection by the piezo injector is determined in accordance with the time when the piezo stack contracts by an arbitrary degree. Therefore, the end timing of the injection is changed from the time point te to a delayed time point te′, and an injection period changes in accordance with the change of a discharging period. As a result, actual quantity of the fuel injected into combustion chambers of the respective cylinders is deviated largely from command injection quantity.
Other than the method for changing the charging amount to the piezo stack in the two levels, there is another method for changing charging speed of the charging voltage applied to the piezo stack as shown in FIG. 9. In this case, the charging amount to the piezo stack changes in accordance with the change in the charging speed. Accordingly, the discharging waveform changes with respect to the time, and the charging waveform also changes with respect to the time. Thus, the injection end timing is changed from the time point te to an advanced time point te′. In addition, the injection start timing of the piezo injector is changed from the time pint ts to a delayed time point ts′. As a result, the actual quantity of the fuel injected into the combustion chambers of the cylinders is deviated largely from the command injection quantity. Moreover, emission performance or drivability performance of the engine is also deteriorated.